1. Field of the Invention
The present invention relates to a fuel cell including an electrolyte electrode assembly and a pair of separators. The electrolyte electrode assembly includes an anode, a cathode, and an electrolyte interposed between the anode and the cathode. The electrolyte electrode assembly is interposed between the separators.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which comprises two electrodes (anode and cathode) and an electrolyte membrane interposed between the electrodes. The electrolyte membrane is a polymer ion exchange membrane (proton exchange membrane). Each of the electrodes comprises a catalyst and a porous carbon sheet. The membrane electrode assembly is interposed between separators (bipolar plates). The membrane electrode assembly and the separators make up a unit of the fuel cell (unit cell) for generating electricity. A plurality of unit cells are connected together to form a fuel cell stack.
In the fuel cell of the fuel cell stack, a fuel gas such as a hydrogen-containing gas is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions (protons) and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electric current. An oxygen-containing gas or air is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
For example, as shown in FIG. 7, a fuel cell stack 10 is mounted on a base 1 such as a vehicle body of a vehicle. In the fuel cell stack 10, a plurality of unit cells (fuel cells 12) are connected together in series, and stacked horizontally (in a direction indicated by an arrow A) to form a stack body 13.
The unit cell 12 includes a membrane electrode assembly 20 and separators 22a, 22b. The membrane electrode assembly 20 includes an anode 14, and a cathode 16, and an electrolyte membrane 18 interposed between the anode 14 and the cathode 16. The membrane electrode assembly 20 is interposed between the separators 22a, 22b. Each of the separators 22a, 22b has a first reactant gas flow passage 24 on its surface facing the anode 14, and a second reactant gas flow passage 26 on its surface facing the cathode 16. A fuel gas such as a hydrogen-containing gas is supplied to or discharged from the anode 14 through the first reactant gas flow passage 24. An oxygen-containing gas such as air is supplied to or discharged from the cathode 16 through the second reactant gas flow passage 26.
Terminal plates 34a, 34b are electrically connected to the outermost unit cells 12 disposed at opposite ends of the stack body 13 in the stacking direction, respectively. Insulator plates 36a, 36b for prevention of electric leakage are stacked on the outside of the terminal plates 34a, 34b, respectively. End plates 38a, 38b are stacked on the outside of insulator plates 36a, 36b, respectively. Further, back up plates 40a, 40b are disposed outside the end plates 38a, 38b, respectively. The unit cells 12, the terminal plates 34a, 34b, the insulator plates 36a, 36b, end plates 38a, 38b, and back up plates 40a, 40b make up the fuel cell stack 10.
A plurality of spring members 42 such as belleville springs are interposed between the end plate 38a and the back up plate 40a for maintaining electrical connections between the adjacent unit cells 12.
In the peripheral area of the fuel cell stack 10, a plurality of through holes 44 are formed. The through holes 44 extend from the back up plate 40a to the other back up plate 40b. Tie rods 46 are inserted into the through holes 44, respectively. Nuts 48 are threaded over the tie rods 46 to tighten the back up plates 40a, 40b. Therefore, the stack body 13, the terminal plates 34a, 34b, and the end plates 38a, 38b are pressed together, and the belleville springs 42 are compressed.
The fuel cell stack 10 is mounted on the base 1 using brackets 50, 52. The brackets 50, 52 are connected to the end plate 38a, the back up plate 40b, respectively. The bracket 52 is fixed to the base 1 by a bolt 54. The bracket 50 is slidable on the base 1.
An arm 56 extends from a lower end of the bracket 50. An oblong groove 60 having a step 58 is formed in the arm 56. A bolt 62 is inserted in the oblong groove 60. The step 58 is pressed by a head of the bolt 52 with a suitable force. In this manner, the bracket 50 is slidably mounted on the base 1.
When the stack body 13 is thermally expanded in the stacking direction during the operation of the fuel cell stack 10, the belleville springs 42 are compressed to some extent corresponding to the amount of thermal expansion. When the operation of the fuel cell stack 10 is stopped, and the temperature of the fuel cell stack 10 is lowered, the stack body 13 is thermally contracted. Therefore, the belleville springs 42 are stretched. The thermal expansion or contraction of the stack body 13 causes the belleville springs 42 to be compressed or stretched. Therefore, the tightening force applied to the stack body 13 is maintained substantially.
The electrolyte membrane 18 absorbs and releases water produced in the electrochemical reaction. Further, the electrolyte membrane 18 absorbs and releases moisture in the fuel gas and the oxygen-containing gas. Therefore, the electrolyte membrane 18 swells and shrinks in the stacking direction of the stack body 13. Further, the membrane electrode assembly 20 wears out with thermal changes in the repeated use of the fuel cell stack 10. Therefore, the rigidity of the membrane electrode assembly 20 is reduced, and the size of the membrane electrode assembly 20 is reduced slightly. The size reduction also occurs in a sealing member (not shown) for supporting the membrane electrode assembly 20, and the separators 22a, 22b. 
In the fuel cell stack 10, when the dimension of the components such as the electrolyte membrane 18, the sealing member, the separators 22a, 22b changes in the stacking direction, the belleville springs 42 are compressed or stretched correspondingly. Therefore, the stack body 13 is constantly pressed together under a desirable pressure.
In the fuel cell stack 10, when the dimension of the components such as the electrolyte membrane 18, the sealing member, the separators 22a, 22b changes in the stacking direction, the belleville springs 42 are compressed or stretched correspondingly. Therefore, the stack body 13 is constantly pressed together under a desirable pressure.
When the dimension of the components changes in the fuel cell unit 10, and the belleville springs 42 are compressed or stretched, the bracket 50 guided by the oblong groove 60 and the bolt 62 slides on the base 1 in the stacking direction.
In the fuel cell stack 10, the bracket 50 is slidably mounted on the base 1. Therefore, a relatively large space is needed for installing the fuel cell stack 10. Normally, the separators are made of carbon, and relatively thick to have a necessary strength. A plurality of the belleville springs 42 are interposed between the end plate 38 and the back up plate 40a. The fuel cell 10 has a relatively large dimension due to the belleville springs 42. Therefore, the overall fuel cell stack 10 is relatively large and heavy.